Phase-Engineering-Induced Generation and Control of Highly

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Letter

Phase-Engineering-Induced Generation and Control of Highly Anisotropic and Robust Excitons in Few-Layer ReS

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Shuyi Wu, Yun Shan, JunHong Guo, Lizhe Liu, Xiaoxu Liu, Xiaobin Zhu, Jinlei Zhang, Jiancang Shen, Shijie Xiong, and Xinglong Wu J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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The Journal of Physical Chemistry Letters

Phase-Engineering-Induced Generation and Control of Highly Anisotropic and Robust Excitons in Few-Layer ReS2

Shuyi Wu1,†, Yun Shan1,2,†, Junhong Guo3,†, Lizhe Liu1,*, Xiaoxu Liu1, Xiaobin Zhu1, Jinlei Zhang1, Jiancang Shen1, Shijie Xiong1, and Xinglong Wu1,*

1

Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics and Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China.

2

Key Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, People’s Republic of China. 3

School of Optoelectronic Engineering and Grȕenberg Research Centre, Nanjing

University of Posts and Telecommunications, Nanjing 210023, People’s Republic of China.

†

These authors contributed equally to this work.

Abstract The anisotropic excitons behavior in two-dimensional materials induced by spin-orbit coupling or anisotropic spatial confinement has been exploited in imaging applications. Herein, we propose a new strategy to generate high-energy and robust anisotropic excitons in few-layer ReS2 nanosheets by phase engineering. This approach overcomes the limitation imposed by the layer thickness enabling production of visible polarized PL at room temperature. Ultrasonic chemical exfoliation is implemented to introduce the metallic T phase of ReS2 into the few-layer semiconducting Td nanosheets. 1

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In this configuration, light excitation can readily produce “hot” electrons to tunnel to the Td phase via the metal/semiconductor interface to enhance the overlap between the wave functions and screened Coulomb interactions. Owing to the strong electron-hole interaction, significant increase in the optical band gap is observed. Highly anisotropic and tightly bound excitons with visible light emission (1.5 − 2.25 eV) are produced and can be controlled by tailoring the T phase concentration. This novel strategy allows manipulation of polarized optical information and has large potential in optoelectronic devices. TOC GRAPHICS

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Current optoelectronic devices composed of two-dimensional (2D) materials are mainly based on the band gap as the optical response to encode electronic information in which photoluminescence (PL) and absorption are two important spectroscopic techniques to realize this goal.1-5 More efficient devices for optical information storage and processing can be produced if the degree of freedom of PL polarization is also introduced.6-8 Such an optical device requires the generation and control of PL polarization. That is, the PL needs to display polarized characteristics and is in the visible region. For example, because of the absence of the inversion symmetry in the monolayered MoS2 structure, valley polarization induced by the spin-orbit coupling effect is adopted to obtain circularly polarized PL in the visible region.1,2 However, this performance is strongly related to the MoS2 layer thickness because of the crossover from the direct band gap of the monolayer to the indirect band gap of the multilayer. To overcome this difficulty, linearly polarized light has been used to directly excite anisotropic 2D materials8 to generate excitons with particular spatial confinement and to manipulate the PL behavior. Although this polarization strategy slightly depend on the thickness of the few-layer materials, the PL energies deviate from the visible region.7,8 With regard to 2D transition-metal dichalcogenide semiconductors (TMDCSs) with a considerable band gap (Eg ~ 1 to 2 eV), the symmetric hexagonal structure cannot produce anisotropic excitonic emission by anisotropic quantum confinement.4,9-12 Although monolayered black phosphorus has an anisotropic symmetry, the polarization-resolved PL from anisotropic bright exciton (1.3 eV) is beyond the visible range due to the relatively small band gap.8 Unlike other hexagonal 2D TMDCSs,9-12 3



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layered ReS2 crystallizes from the distorted 1T diamond-chain structure (Td phase) with a triclinic symmetry as a result of charge decoupling from an extra valence electron of the Re atom.13-19 The structural distortion gives rise to much weaker interlayer coupling and hence, band renormalization is absent and bulk ReS2 behaves as an electronically and vibrationally decoupled monolayer. 13,18,19

The vanishing interlayer coupling is

advantageous to controlling the exciton emission behavior by regulating the carrier interaction in the excited states.

Figure 1. Scheme for phase engineering and detection of visible polarized excitonic emission. (a) Diagrammatic representation of the PL mechanism in the Td-ReS2 nanosheets. (b) T phase incorporation strategy to achieve a larger band gap in the T@Td-ReS2 interfacial region. The pink arrows show the built-in electric field direction. (c) Anisotropic wave function of the exciton and detection of polarization-resolved PL with the red arrows showing the emission intensity.

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A schematic of the PL mechanism in the Td phase ReS2 structure is presented in Figure 1a. Absorption of the laser pulse promotes the transition of electrons from the valence band (VB) to conduction band (CB), followed by relaxation to the exciton binding energy (Eb) level via a non-radiative recombination process to generate ∼ 1.5 eV excitonic emission (Eo = Eg − Eb).7,14,17-19 However, this emission is still in the non-visible region and cannot be easily controlled by changing the Td ReS2 layer number because of weak interlayer coupling (Figure S1 in Supporting Information).

If

external electrons are intentionally implanted to adjust the electron-electron (e-e) and electron-hole (e-h) interactions, the electron effects will enlarge the band gap producing excited-state properties.16 Figure 1b shows the schematic of the centro-symmetric T phase ReS2 (T-ReS2) in the Td ReS2 matrix forming an effective interfacial region (T@Td-ReS2). T-ReS2 has a metallic behavior19 and more importantly, electrons in T-ReS2 can easily transfer to Td-ReS2 during light excitation due to the photothermal effect.20,21 Therefore, the incorporated T-ReS2 not only provides the necessary excited-state electrons, but also generates a built-in electric field to adjust the electron-hole (exciton) recombination process. By increasing the T-ReS2 concentration, more robust anisotropic excitons can be produced and controlled by implanting more external electrons and constructing stronger built-in electric field to enhance the anisotropic quantum confinement effect.

Consequently, excitonic recombination can

produce angle-dependent visible PL (Figure 1c). The T phase incorporated Td-ReS2 nanosheets are prepared by the ultrasonic chemical exfoliation method (see Sample Preparation and Characterization in 5



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Supporting Information). The critical step is to control the T-ReS2 nanocrystal concentration in the Td-ReS2 matrix. The initial Td-ReS2 single crystals are produced by the Br2-assisted chemical vapor transport method.12 The Td-ReS2 crystals are sonicated in nitric acid at room temperature several hours to introduce the desired Re vacancies which act as distortion sites.

At the same time, the thermal energy produced facilitates

the formation of the meta-stable states. Finally, Peierls distortion in which local neighboring Re atoms move away from lozenge of four atoms into the metal sites of the octahedral T structure occurs.22

Figure 2. Characterization of the T@Td-ReS2 nanosheets. (a) SEM image. (b) AFM image. Inset: Height profile along the white arrow indicating the layer thickness. (c) HR-TEM image. (d) Atomic structural model at the phase interface. The images consisting of blue and red balls indicate the symmetry of the Td and T phases, and the yellow and green balls represent the S and Re atoms, respectively. 6


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Figure 2a displays the scanning electron microscopy (SEM) image of the T@Td-ReS2 nanosheets which are rectangular and have a planar size of 4.5 − 13.5 µm. Figure 2b shows the typical atomic force microscopy (AFM) image disclosing changes in the thickness.

The inset reveals that the thickness of the edge of a T@Td-ReS2

nanosheet is about 0.9 nm (monolayer) to 1.7 nm (bilayer). Generally, the nanosheet thickness varies from 0.8 to 8 nm (1-10 layers with an interlayer spacing of ~0.75 nm). High-resolution transmission electron microscopy (HR-TEM) reveals that the T and Td phases can coexist in a nanosheet (Figure S2 in Supporting Information) and Figure 2c clearly shows the interfacial region between the two phases. The lattice fringes of 0.285 and 0.296 nm can be indexed to the (100) planes of T-ReS2 and Td-ReS2, respectively, suggesting formation of a lateral interfacial heterostructure in the Td-ReS2 nanosheet. X-ray diffraction, X-ray photoemission, and Raman scattering corroborate the conclusion (Figures S3-S5 in Supporting Information). The interfacial structure composed of the T and Td phases is also theoretically simulated as shown in Figure 2d (please refer to the calculation details in Supporting Information). Different from the hexagonal symmetrical structure, Td-ReS2 shows a distorted T structure with clustering of Re4 units forming parallel metal chains along the van der Waals plane12 consistent with the HR-TEM results. The room-temperature PL spectra in Figure 3a provide information about generation of excitonic emission from the T@Td-ReS2 nanosheets. Compared to the PL (peaked at 1.51 eV) of the pristine Td-ReS2 nanosheet (Figure S1 in Supporting Information), the PL peaks blue-shift significantly to 2.05 eV (visible region). The peak 7



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positions depend slightly on the layer number (Figure 3b) being consistent with previous results obtained from few-layer ReS2 flakes (Exp-Td).7,17-19 The large blueshift from 1.51 to 2.05 eV (Exp-T@Td) indicates that the introduced T-ReS2 plays a critical role in the excitonic emission energy. To explain the large PL blueshift, we theoretically calculate the e-h and e-e interactions in the excited states by the many-body perturbation theory (Cal-GW) based on the density functional theory (Cal-DFT) (see the calculation details in Supporting Infromation) and find that the blueshift can indeed occur in the T@Td-ReS2 heterostructure (Figure 3b).

Figure 3. Measured PL spectra and theoretical interpretation for the T@Td-ReS2 nanosheets. (a) PL spectra acquired from sample S3 in the different layer thickness regions. (b) Comparison between the calculated and experimental results for different layer thicknesses. (c) PL spectra obtained from different T phase concentrations in the T@Td-ReS2 nanosheets.

To determine the role of T-ReS2 in the excitonic behavior, PL spectra for different T phase concentrations (changing the average distance between the T-ReS2 regions and T-ReS2 phase sizes as discussed in Sample Preparation and Characterization and Figures S6a and S6b in Supporting Information) are obtained as shown in Figure 3c.

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When some Td phases are converted into the T phase to form T@Td-ReS2 (sample S1), the emission peak begins to blue-shift from 1.51 (pristine Td-ReS2) to 1.63 eV. As the T-ReS2 concentration is increased further, the emission peaks expand into the visible region (1.77 eV for sample S2; 2.06 eV for sample S3; 2.25 eV for sample S4). Obviously, this cannot be attributed to impurities/contaminants, because there is no related PL from the nitric acid solution (see the IR spectrum in Figure S7 in Supporting Information). In addition, it is known that the vacancies and impurities can only produce red-shifts in the PL peak (Figure S8 in Supporting Information).

Figure 4. Measured PL spectra and angle-dependent polarized characteristics of the T@Td-ReS2 nanosheets. (a) Polarization-resolved PL spectra acquired from sample S3 revealing the excitonic nature of emission. (b) PL peak intensity as a function of polarization detection angle for the excitation laser polarized along the x and y directions for 9



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sample S3. (c) PL excitation intensity map as a function of the excitation and emission photon energies. (d) Calculated exciton Bohr radius to display the anisotropic electron wave function distribution and Coulomb interaction.

To investigate the excitonic feature, the polarized PL spectra of the T@Td-ReS2 nanosheets are depicted in Figures 4a and 4b (samples S3 as an example). The polarization-resolved PL measurements are described in Supporting Information. The excitation (marked by E) and detection (marked by D) polarizations are selectively oriented along either the x or y axis corresponding to the shortest and second-shortest axes in the basal plane (marked by black arrows on the left side of Figure 2d). The four different measurements all show a single peak at 2.05 eV and so the peak position is independent of excitation or detection polarization. However, the largest PL intensity is observed when both excitation and detection polarization are aligned with the x direction (E-x; D-x). When it changes to the y direction, the PL intensity (E-x; D-y) is obviously attenuated by more than 70% compared to that along the x direction (E-x; D-x) regardless of the excitation light polarization. To show the polarized feature more clearly, the angle-dependent PL intensity is shown in Figure 4b in which the excitation polarization is arranged along the x and y directions. Because the wave function is strongly extended along the 0o and 180o (x) direction, observation of highly polarized emission indicates an anisotropic excitonic nature. The excited electrons and holes are preferentially dispersed along the x band direction and the isotropic Coulomb interaction leads to stronger binding effects of e-h in the y direction. Furthermore, in the T@Td-ReS2 interfacial region, the exciton recombination process is adjusted by the 10


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built-in electric field induced by the incorporated T-ReS2. Without considering excitation light polarization, higher energy photons will first generate “hot” electrons in T-ReS2 and free elections and holes in Td-ReS2 and then the “hot” electrons are transferred to Td-ReS2 to control the e-h interaction in which anisotropic excitons form to produce highly polarized light emission (Figure S9 in Supporting Information). Thus, regardless of the excitation polarization, the emitted light is strongly angle-dependent because the PL always originates from recombination of these anisotropic excitons. Similar polarized features are observed from the samples with different T-ReS2 concentrations (Figure S10 in Supporting Information). These results showing strongly polarized PL are consistent with our theoretical prediction (Figure 1) that shows that excited states are dominated by anisotropic excitons as a result of the structural symmetry and screening effect. Two-dimensional mapping of the PL excitation (PLE) spectra as functions of both excitation and emission photon energies is shown in Figure 4c (see Sample Preparation and Characterization in Supporting Information). The strongest PL occurs at an excitation energy of 2.45 eV (white dotted lines), which is four times stronger than that at an excitation energy of 2.14 eV. The PL intensity as a function of excitation energy (the black dotted line) is plotted in Figure S11 in Supporting Information. The optical band gap in the excited state increases as 2.45 eV. To reveal the anisotropy of the PLE spectra, the exciton Bohr radius is calculated (Figure 4d) to show the electron wavefunction distribution (Figure S12 in Supporting Information for calculated effective mass). The asymmetrical distribution in the wavefunction implies that the 11



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recombination probability of excited electrons and holes depends on spatial confinement which causes the observed anisotropic emission of excitons. The schematic presentation of the “hot” electron transfer process based on the interfacial band structure is shown in Figure 5a. The difference in the work function between the metallic T-ReS2 and semiconducting Td-ReS2 is only 0.12 eV thereby causing a small potential barrier in the phase interface.

During light excitation, many

Figure 5. “Hot” electron transfer mechanism and calculation of exciton binding energy. (a) Schematic representation of the “hot” electron transfer process based on the calculated results. (b) Top: Calculated excitonic absorption with e-h interactions in the different deformed structures (T1: compressed 2%; T2: original structure; T3: stretched 2%). Bottom: Calculation quasiparticle absorption and differential curves for the T2 structure. The 12


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estimated quasiparticle band edge is indicated by the shaded region (theoretical) and red line (experimental).

excited “hot” electrons (free electrons) of T-ReS2 can readily penetrate the Td-ReS2 region by tunneling with a transmission coefficient of about 85% (see Supporting Information for calculation details). Electron accumulation in the Td-ReS2 produces the many-electron effect and excited-state properties, which not only increase the optical band-gap, but also modify the PL process with the built-in electric field. The metal-semiconductor contact increases the effective transfer range of “hot” electrons to reach 25 nm (see Supporting Information for calculation details). Since the average distance between the T-ReS2 phase regions is about 9 nm (see the HR-TEM image of Figure S2 in Supporting Information), electron transfer occurs easily. To determine the exciton binding energy, the quasiparticle absorption spectrum and differential results (Dif) are derived and shown in the lower panel of Figure 5b. In general, the maximum in the differential absorption spectrum is the quasiparticle bandgap. According to our experiments, the optical band gap in the excited state can be extracted as 2.45 eV (marked by red shade line in Figure S11 in Supporting Information), which is in accordance with the calculated quasiparticle band gap (2.43 eV in the lower panel of Figure 5(b)). After considering the excitonic effect by solving the GW-Bethe-Salpeter equation,23,24 the absorption peak shifts to 1.98 eV (T3: which is consistent with the measured absorption spectra of T@Td ReS2 in Figure S13 in Supporting Information) leading to an exciton binding energy of 0.45 eV. However, two minor factors need to be considered when comparing the experimental and theoretical 13



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results. Firstly, the PL behavior of the T@Td-ReS2 structure depends lightly on the layer thickness and so the excited-state property of only the monolayer Td-ReS2 structure is considered in the calculation. Secondly, the influence of the T phase concentration on the Td-ReS2 PL behavior can be understood as changes in the overlap between the wave function and screened Coulomb interactions as described by screened-exchange interaction (see Supporting Information for calculation details). When the original space distance is compressed about 2% (T1: as shown in upper panel of Figure 5b), spatial localization of electronic states is enforced giving rise to a lager quasiparticle bandgap. On the contrary, the relaxed space structure (about 2%) (marked by T2) lessens the overlap between the wave function and screened Coulomb interaction producing a smaller bandgap. This trend of the change in excitonic absorption with spatial restriction is similar to that observed experimentally. In conclusion, the T phase incorporated in the Td-ReS2 host cannot be simply regarded as a secondary phase because the embedded metallic T-ReS2 phase alone cannot cause the PL to shift to the visible region, but rather provides “hot” electrons to Td-ReS2 and forms a built-in electric field to restrict the excitonic behavior of Td-ReS2. Only those excited electrons and holes located in the effective region of the “hot” electron distribution (around T phase interface) can interact with each other in an excited-state manner to produce the observed visible polarized excitonic emission from the T@Td-ReS2 nanosheet. This phase-engineering strategy does not depend on the ReS2 thickness thereby allowing us to probe the PL of 2D-like systems without the need to prepare large-area, single-crystal monolayers. This idea can be generalized to other 14


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2D materials to tune the optical properties.

ASSOCIATED CONTENT Supporting Information available: Sample

preparation

and

characterization,

PL

and

polarization-resolved

PL

measurements, theoretical calculation, HR-TEM images, XRD patterns, XPS spectra, Raman spectra, IR spectra, calculated band structures, PL decay curves angle-dependent polarized characteristics, PL intensity as a

function of excitation energy,

angle-dependent effective masses of electron (me), hole (mh), and excition (m). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] (L.Z.Liu), [email protected] (X.L.Wu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Basic Research Programs of China under Grants Nos. 2014CB339800 and 2013CB932901, National Natural Science Foundation (Nos. 11374141, 61521001, 11674163, 61505085, and 11404162), and Natural Science Foundation of Jiangsu Province (BK20161117). Partial support was from High 15



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Performance Computing Centers of Nanjing University and Shenzhen.

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